Definitions

• Definition Real-Time Systems:
  A Real-Time System responds in a (timely) predictable way to unpredictable external stimuli arrivals. In short, a Real-Time System has to fulfil under extreme load conditions:
  1. timeliness: meet deadlines, it is required that the application has to finish certain tasks within the time boundaries it has to respect.
  2. simultaneity or simultaneous processing: more than one event may happen simultaneously, all deadlines should be met.
  3. predictability: the real-time system has to react to all possible events in a predictable way.
  4. dependability or trustworthiness: it is necessary that the real-time system environment can rely on it.(dixit Martin Timmerman)

• Other definitions
• Classification and examples:
  • Dedicated Systems
  • Real-Time Systems
  • Hard Real-Time Operating Systems
  • Soft Real-Time Operating Systems
  • Other Real-Time Systems Examples
  • Embedded Systems
  • Fault-tolerant systems

Dedicated Systems
A system where the functionality is once and for all tied up into the hard- and software. Most dedicated systems have soft real-time constrains, others need to be hard real-time and some have fault-tolerance build in.

Real-Time Systems
A system is a real-time system when it can support the execution of applications with time constraints on that execution. There can be made a classification into hard and soft real-time systems based on their properties, each of them is explained with the specific example.

An example of a hard real time system is a digital fly-by-wire control system of an aircraft:
No lateness is accepted under any circumstances, otherwise the aircraft is not controllable.
Useless results if late, if the control system does not respond timely, the result is a hole in the ground.
Catastrophic failure, which needs no explanation in the case of an aircraft crash.
Cost of missing deadline is infinitely high, the lives of people depend on the correct working of the control system of the aircraft.

A soft real-time system can be a vending machine:
Rising cost for lateness of results: As it will take longer to treat a customer when the performance of the vending machine is degrading, less customers pay at this machine which results in less profits for the shop owner.
Accept lower performance for lateness, it is not catastrophic when deadlines are not met. It will take longer to handle one client with the vending machine.

Other real-time systems examples are nuclear power plant control, industrial manufacturing control, medical monitoring, weapon delivery system, space navigation and guidance, reconnaissance systems, laboratory experiments control, automobile engines control, robotics, telemetry control systems, printer controllers, anti-lock breaking, burglar alarms,... The list is endless.
Embedded systems:
Computer system that is enclosed in another system and makes an essential part of it. OR. Hardware and software, which forms a component of some larger system and which, are expected to function without human intervention.
Fault-tolerant systems:
Systems that continue working in all circumstances (except for physical destruction).
OR.
The ability of a system or component to continue normal operation despite the presence of hardware or software faults.